1. Field of the Invention
The present invention relates to a mobile communication system in the multi-carrier CDMA (Code Division Multiple Access) scheme.
2. Description of the Related Art
The CDMA scheme in which each communicating party is identified by using a spreading code allocated to each communicating party and a plurality of communicating parties carry out communications using the identical frequency band has been known conventionally. The next generation mobile communication scheme called IMT-2000 adopts a radio access scheme called wideband direct spreading (DS) CDMA scheme (which will be referred to as W-CDMA scheme hereafter) which uses the spreading bandwidth of 5 MHz or more.
In the downlink of this W-CDMA scheme, each communicating party is identified by using a short code having a repetition period equal to a data symbol period, which is a spreading code allocated to each communicating party at a radio base station. On the other hand, a radio mobile station identifies each radio base station by using a long code having a much longer repetition period compared with the short code.
FIGS. 1A and 1B show the conventional spreading code allocation methods in the downlink of an inter-cell asynchronous system and an inter-cell synchronous system, respectively. As shown in FIG. 1A, the W-CDMA scheme adopts the inter-cell asynchronous system that requires no external system for the purpose of the timing synchronization, in which long codes #0, #1 and #2 that are different for different radio base stations are used in order to identify radio base stations of respective cells 104, 106 and 108 at a long code layer 100. Note that the long code is also referred to as a scrambling code in a sense that it scrambles signals from the other codes as noises.
On the other hand, the cdma2000 scheme that has been proposed in the United States as a candidate for the IMT-2000 instead of the W-CDMA scheme or the conventionally known IS-95 scheme realizes the inter-cell synchronous system as shown in FIG. 1B, which uses a GPS 116 or the like to provide a timing reference common to all the radio base stations 110, 112 and 114 at the long code layer 102. In this system, the radio base stations are identified by using a single type of a long code to which different timing shifts #0′, #1′ and #2′ are given.
Now, as a radio access scheme of the mobile communication system after the IMT-2000, the adoption of a method for transmitting signals by using multiple carriers such as a multi-carrier DS-CDMA scheme or a multi-carrier CDMA scheme is currently under the discussion. Here, the multi-carrier CDMA scheme is a transmission scheme which transmits signals by using a plurality of sub-carriers by arranging copies of the data symbol on a frequency axis and multiplying each of them with the spreading code on that frequency axis. In this multi-carrier CDMA scheme, a plurality of communicating parties will carry out communications by using the identical frequency band simultaneously.
However, the discussion regarding the multi-carrier CDMA scheme so far has been mainly focused on the discussion of the performance evaluation at the link level and the timing and frequency synchronization. Although the multi-carrier CDMA scheme also identifies the communicating party by using the spreading code allocated to each communicating party similarly as in the conventional DS-CDMA scheme, there has been no discussion of a method for efficiently allocating the spreading code conventionally.
Also, in the case of using the multi-carrier CDMA scheme as the mobile communication scheme, although there is a need to identify radio base station just as in the case of using the W-CDMA scheme, there has been no discussion of a method for realizing this.